We also see the BoP flying with wings down when it's attacking with it's wing disruptors, but wings flat out when cruising at warp speed. Perhaps the flat wings aid in the efficiency of the warp drive and the lower configuration somehow makes the weapons more powerful...though how, I couldn't guess. We see in ST5 that they can steer left and right, so maybe they are lower so that they don't risk hitting their own bridge....?